Methods and systems for treating tissues using heat therapy or hyperthermia are well known. For example, the treatment of cancerous tissue with heat is know to be beneficial. Specifically, heating such abnormal tissue above a threshold temperature of approximately 42.degree. Celsius introduces irreversible cellular damage to the tissue. In this manner, benign and malignant tumors may be necrosed. At temperatures above 45.degree. Celsius, irreversible damage occurs not only to abnormal tissue but to healthy tissue as well. The temperature range of approximately 42-45.degree. Celsius is known as the therapeutic hyperthermic range for human tissue. Heat therapy in the range of 45-60.degree. Celsius is generally known as thermal therapy or thermotherapy. At temperatures above 65.degree. C., tissue is ablated.
While the generally recognized temperature range for necrosing abnormal tissue is well known, a number of other factors may affect the influence of heat on specific tissue. For example, if the tissue has an abnormally low cellular pH, poor oxygenation, or nutritional deprivation, the tissue is more likely to be vulnerable to irreversible damage at temperatures lower than normally expected. This is one of the reasons that abnormal or neoplastic tissues are thought to be more susceptible to temperature increases than normal tissue.
To supply heat to abnormal tissue to hyperthermally treat the tissue, both external noninvasive and Minimally Invasive Surgery ("MIS") techniques and technology have been developed. Non-invasive induction techniques use radiofrequency (RF) and microwave applicators as well as ultrasound transducers. The MIS techniques and technology largely involve inserting therapy probes into the body which emit energy directed to a target area of tissue to destroy the diseased tissue. The energy types emitted by the probes are typically laser, microwave, RF, electrocautery, or ultrasound in nature. These probes are placed proximate the abnormal target tissue for administration of the energy either through interstitial, intracavitary or laproscopic methods. Interstitial placement of a probe means that the probe, usually introduced by a needle or catheter, is placed directly into the tissue. An intracavitary method requires that the probe be placed in a natural body opening proximate the target tissue and laproscopic surgery requires a small incision which permits the placement of the probe proximate the target tissue. For example, a thermal therapy probe may be used to treat a prostate cancer by inserting the probe through the perineum or urethra into the diseased area (interstitial), into either the urethra or rectum and directed toward the target area (intercavitary), or through a laproscopic incision to allow placement of the probe near the tissue. Such exemplary methods may be used to place probes for the emission of energy into abnormal tissues to treat, for example, non-resectable liver cancer, prostate cancer, pancreatic cancer, breast cancer and various gynecological diseases.
While these MIS techniques permit energy emitting probes to be placed proximate the tissue to be treated so that the tissue contacted by the emitted energy is heated, there has been little provision for the monitoring of temperature either at the target area or in the neighboring areas. As a result, a treating physician may have difficulty in determining whether an adequate thermal dosage has been applied to the target area to sufficiently necrose or ablate the abnormal tissue without affecting the surrounding normal tissue. If the treating physician fails to supply a sufficient dosage of a thermal treatment to destroy all of the abnormal tissue, then the surviving abnormal tissue may regenerate and the cancer or other abnormal cellular condition may continue. On the other hand, if the physician applies more than an effective dosage of the thermal treatment, then the amount of normal tissue affected by the treatment may increase morbidity or cause side effects which adversely impact the patient's health.
In response to this need to effectively determine the dosage of the thermal treatment, various attempts have been made to measure the thermal dosage or its effects. One such attempt has been to time the emission of the energy which generates the thermal effect in the tissue for a predetermined period of time. This timing constraint, however, relies upon assumptions about both the abnormal tissue and the condition of the normal tissue surrounding the target area. To compensate for different tissue conditions caused by age, the patient's general health, oxygenation of the surrounding tissue, blood perfusion in the surrounding tissue, and other known parameters which affect the energy absorption rates and therapeutic dosage achieved in the target tissue, empirical studies are necessary. These studies are used to provide general parameters which, at best, can only provide estimated times for a thermal dosage. Thus, use of a predetermined time to control the dosage of the thermal treatment to a target area in the absence of adequate monitoring of the treatment may result in ineffective treatment of the abnormal tissue or may result in the unnecessary destruction of healthy tissue.
In an effort to supply some feedback of temperature data from the target area and surrounding area, other methods for monitoring tissue temperature have been developed. One such method requires the implantation of temperature probes in the tissue to provide information regarding temperature in the affected tissue area. Placement of these temperature probes is a delicate operation and knowledge of the exact location of the probes and sensors is essential to accurately discern temperatures within the target tissue and outside the target region as well as to extrapolate temperatures for the area surrounding the probes and sensors. Additionally, the placement of these probes requires invasive techniques and further requires the removal of these probes after treatment. Of course, these invasive techniques and the placement of the temperature probes within the tissue add to the invasive nature of the overall procedure and may cause trauma from which the patient must heal. Additionally, heat is conducted from the treated area outwardly to the surrounding tissue at different rates primarily due to differing perfusion levels and energy absorption rates by different tissue types. The effective monitoring of the heating within and surrounding the treated area may require a number of probes which a treating physician may not consider practical or clinically acceptable. Another factor affecting the effectiveness of temperature probes is the different thermal conductivity of healthy tissue versus that of abnormal tissue. Normally, the temperature probes are placed in the healthy tissue surrounding an affected area. As a result, the rate of transfer of heat from the target area to the healthy tissue area may cause the thermal probe to not register a higher temperature until tissue between the probe and the abnormal tissue area has already been destroyed. Thus, the location of temperature probes within the normal tissue is important and may affect the effectiveness of the thermal treatment.
Another attempt to provide temperature monitoring during thermal therapy techniques includes the use of magnetic resonance imaging to display temperature changes in tissue. Magnetic resonance imaging techniques are typically cost prohibitive as they require the use of special non-magnetic instruments and facilities to perform such procedures. Accordingly, the use of such technology appears to be limited to very special environments, instrumentation, and to procedures which require imaging of relatively large areas.
Finally, some attempts have been made to utilize ultrasound imaging for thermal therapy monitoring. Ultrasound data is typically expressed in grayscale pixel data which is used to image tissue structure in various shades of gray. The changes in tissue structure as abnormal tissue is necrosed or ablated are not significant enough to perceptually change the displayed image. In a similar manner, the structural change within normal tissue as its temperature approaches the destructive threshold where universal cellular damage occurs, may not be directly observable in a typical grayscale ultrasound image.
What is needed is a way to monitor the temperature of tissue in a hyperthermic or thermally treated target area and vicinity.
What is needed is a way of monitoring thermal conditions in tissue without requiring placement of numerous temperature probes or the careful placement of such probes in the target and surrounding tissues.
What is needed is a way of monitoring and displaying temperature data for tissue in the area of thermally treated tissue which does not require cost-prohibitive magnetic resonance instrumentation and environment.
What is needed is a way of monitoring the temperature of tissue with ultrasound energy which provides temperature data or tissue damage information even though the structural characteristics of the tissue are not visually perceptible or significantly altered by the temperature change occurring in the tissue.